This invention relates in general to a television system for transmitting pictures from a transmitter to a receiver and, more particularly, to such a television transmission system which asynchronously and sequentially transmits television pictures as a plurality of video signals over a narrow bandwidth medium with improved synchronization of the receiver to the transmitter, identification of the transmitter, identification of the start of video information, and restoration of the gray scale of the received picture.
Transmission and reception of good quality television pictures by commercial television stations is now a commonplace phenomena. However, to achieve a good quality picture having flicker-free movement and a satisfactory number of gray levels requires a substantial bandwidth for transmission of the video signals between the transmitter and receiver, for example, generally at least a few megahertz.
Many applications exist wherein it is not necessary to update the picture information numerous times each second to provide flicker-free movement. That is, updating of the picture once every couple or few seconds may be sufficient. This slower rate of television picture transmission is suitable, for example, in applications such as surveillance of public and commercial buildings, and in mass transit vehicles. This slower television picture display rate may also be adequate in remote monitoring of automobile traffic or in many industrial processes. Transmission of scenes of accidents or crime, as by law enforcement officers, may also take advantage of narrow band television systems. It will be readily understood that in many of these instances that due to the remoteness of the information site, or due to location of the transmitter on moving vehicles or the like, that it may not be possible or practical to have a closed circuit television system because of the difficulty in providing a suitable cable or other link between the transmitter and receiver or in obtaining the necessary bandwidth in the available frequency spectrum.
In recent years, some television systems have been introduced into the marketplace and others have been proposed which utilize narrow bandwidth transmission mediums, such as telephone lines, to transmit television pictures. Some of these systems have been named "slow scan" television. This is often a misnomer inasmuch as the scanning rate of the television set is not itself generally modified. The television set continues to scan at its normal scan rate of typically 15,750 lines per second. What is different is that, since the picture information is being transmitted more slowly, the picture frames are being updated and changed at a slower rate in contrast to the typical 30 picture frames per second that occur with the familiar flickerfree image obtained from commercial television stations.
Many of these narrow band television systems have not dealt with the problems of transmitting a good quality picture over a standard two-way FM radio having a bandwidth of about 3 kHz. Furthermore, many of the narrow band TV systems have been concerned with receiving picture or video signals from only a single transmitter such that identification of the particular transmitter or requesting a picture from one of a plurality of transmitters has also not been of concern. Where information from remote transmitters is asynchronously sent to a receiver, serious synchronization problems between the transmitter and receiver also occur due to the asynchronous nature of the transmissions and also due to the fact that the transmitters will generally be located at varying distances from the receiver which results in different signal phase shifts. It is also well known to those skilled in the art that signal fading occurs due to multipath propagation of transmitted FM signals, especially when the transmitter or receiver are in motion, as in various types of mobile vehicles. Oscillator instabilities in the receiver or transmitter can also cause frequency drifts. Any frequency drift between the transmitter and receiver can result in DC offset of the demodulated FM signal. Where the demodulated FM signal is a video signal to be recovered as a television picture, the DC offset results in a shift in the gray scales in the recovered TV picture which is, of course, undesirable.